1. Field of the Invention
This invention relates to a circuit for measuring off-sets of a rotary shaft for use in a turbine generator and the like.
2. Description of the Prior Art
FIG. 1 shows an off-set measuring circuit which has been disclosed in, for example, Japanese Patent Publication Laid-Open No. 48802/1981. The off-set measuring circuit is composed of a peak-to-peak amplitude detector 1, reset pulse generators 8.sub.a and 8.sub.b each of which detects a projection 7 on a rotary shaft 6 to generate a train of reset signal pulses V.sub.r1 and another train of reset signal pulses V.sub.r2 which are in synchronism with the rotary period of the rotary shaft 6, a third peak-value holding circuit 5 for holding the peak value of each off-set signal V.sub.1 from a second peak-value holding circuit 4 and a signal selector 9 composed of diodes 10 and 11 adapted to select either one of the off-set signal v.sub.1 and an off-set signal V.sub.2 outputted from the third peak-value holding circuit 5 and a resistor 12, namely, the high level signal. The first and second peak-value holding circuits 2 and 4 are reset by the reset signal pulses V.sub.r1 in a state that the third peak-value holding circuit 5 has held each off-set signal in a given measuring period. In addition, the third peak-value holding circuit 5 is reset in a state that the second peak-value holding circuit 4 has held each off-set signal in the next measuring period.
FIGS. 2(a) through 2(f) are waveform charts of signals at individual devices and describe the operation of the off-set measuring circuit shown in FIG. 1. An alternating-current input signal V.sub.in to be inputted to the peak-to-peak amplitude detector 1 is shown in FIG. 2(a). Each of such reset signal pulses V.sub.r1 and V.sub.r2 as illustrated in FIGS. 2(b) and 2(c) is produced in synchronism with the rotary period of the rotary shaft. First of all, the peak-value holding circuit 5, which is holding the peak value of the signal V.sub.1, is reset by the reset signal pulses V.sub.r2, as shown in FIG. 2(e). After the peak-value holding circuit 5 has held the peak value of the signal V.sub.l again, the first and second peak-value holding circuits 2 and 4 are reset by the reset signal pulses V.sub.r1 as illustrated in FIG. 2(d). When each output V.sub.2 is produced from the peak-value holding circuit 5 in which the signal V.sub.1 in the previous period has been held, the peak-to-peak amplitude detector 1 produces an off-set signal V.sub.1 in the next period. The signal selector 9 selects either one of the signals v.sub.1 and V.sub.2, namely, the signal V.sub.2 of a high level for outputting an output V.sub.out as shown in FIG. 2(f). The same operation is repeated whenever the peak-to-peak amplitude of the input signal V.sub.in is reduced. As a result, following the reductions in the amplitude of the input signal, the amplitude of the output V.sub.out is also reduced with a delay corresponding exactly to one period. Thus, the output signal V.sub.out has such an overall waveform as shown in FIG. 2(f).
Conventional off-set measuring circuits have been constructed as described above. Therefore, when the distance between a sensor for detecting off-sets and an object to be measured (for example, a rotary shaft of a turbine generator) varies stepwise due to thermal expansion of a turbine upon measurement of off-sets of the rotary shaft, the off-set value V.sub.A as a DC component of the off-set input signal V.sub.in also varies stepwise as shown in FIG. 3(a). In addition, as the off-set value V.sub.A changes, a high-level portion appears in the off-set output V.sub.out for a time interval corresponding to the interval between each two consecutive measurements of the off-set of the rotary shaft, namely, one period in spite of the fact that the off-set remains constant as clearly illustrated in FIG. 3(f). As a consequence, the prior art circuits are accompanied by the problem that off-sets cannot be measured accurately.